Propeller Inspection, Defects and Repairs

                                                                  Propeller 

Propeller and the stern tube is an independent survey items from the docking survey. The survey  normally includes complete withdrawal of the propeller, shaft and examination of following, as applicable:

· Measurement of wear down;

· Propeller nut and propeller shaft threaded end;

· Cone, key and key way(keyed shafts), including examination by an efficient crack detection method of the after end of the cylindrical part of the shaft and one-third of the length of the taper from the big end;

· Where propeller is fitted by a solid flanged coupling at the after end of the shaft, non-destructive examination of the flange fillet area of the shaft;

· Propeller shaft bearing areas;

· Stern bushes/bearings;

· Shaft sealing arrangements, including lubricating oil system.

Verification the tightness of the propeller hub (propeller hood, fore gland)(shaft)

 Propeller is removed first before removal of tail shaft, Stern tube liner / bearings for checks, reconditioning and seal replacement - fitted last.

Propeller Removal Procedure 

After removal of fair water cone(cement has to be removed before accessing the nuts inside the slots  to remove the cone) and the locking plate, the pilgrim nut is removed, reversed and together with a loose shock ring is screwed back onto the shaft. A strong back is fitted and secured with studs to the prop boss. Oil is now inserted to the system- for expanding the hub (pressure of hub decided basis the temperature of shaft and graph plot- decided by Von-meiss criterion condition- as calculated for the propeller material and fit - given in manual ) , and the pilgrim nut forcing the  loading ring against the strong-back withdrawal studs and propeller shaft. 

Propeller Defects  

1.Mechanical damage in service - Deformation, Cracks on blade, Broken pieces .

2.Corrosion : Electro-chemical attack - due to service conditions - Pitting, de-zincification - resulting in Cracks , breakage and vibration in service.

3.Erosion : Due to cavitation damage

4.Singing  : Resonance  due to local natural frequency and propeller blade tip and vortex shedding frequency at the trailing edge..result of Tip erosion/ breakage.

Propeller singing phenomenon is defined as the resonance between the local natural frequency of the propeller blade tip and the vortex shedding frequency at trailing edge of the blade

1.Visual Inspection after cleaning

Defects observed :

2. Die penetrant test to ascertain minor cracks .

Small cracks are cannot be detected unless staging’s are erected and after cleaning of each blade. Even the hair cracks pose a possibility of expanding and breaking the propeller blade. This Die Penetrant test need not be carried out for the entire surface of the blade, unless warranted –Usual area of check :root to 0.4R. The blade is divided into parts formed by measuring arcs from the centre of the propeller at every 10% of the radius of the propeller, such as 0.4R, 0.7R. The names of the blades are generally assigned as A, B, C, D and E or (1), (2), (3), (4), and (5) in case of five-bladed propeller. The name of each blade is engraved at the root of the blade.Previous history has to be taken into consideration before final repairs. The causes of this damage are contacting with a submerged or floating object or small material defect during manufacturing which developt to hair cracks, and or metal fatigue.

Repair Methods according to propeller area

Repair Methods adopted in Zone B and C only :Welding & build up/ Cutting & Welding/ Hot Faring/ Cold Faring

Weld repair of Cavitation and Cracks

The surface can be smoothed using a grinder, or depending on the position, the blade can be built up by welding,If the  crack is within 0.4 R , then the whole propeller has to be annealed after repair. If the corrosion is severe, the corroded part may be cut out and using the approved material and repaired by welding(same as broken blades as described before). These welding repairs should be entrusted to the propeller manufacturers , or an approved facility

2. Heat or Cold Forming for bent blades

Bend occurs because of impact of the propeller with other objects; they can be detected easily even before the propeller is cleaned

 Depending on the area to be repaired , the methods for welding and faring were adopted as shown above.

Under water Straightening (cold forming by Jacks)

Bent Blades

Diver attaching Hydraulic Straightener

Hot Forming by Heating and Jacking up or groove welding to cause required bend straightening- followed by proper stress relieving.

3.Build up by welding castings

- Broken blades during course of sailing/ allusions during service- Characterised by vibration in stern .In case this breakage is less than one third of the blade length, the build up method is followed

Usual method of repair involves building up of the tip with same cast metal (a sample of propeller metal is analysed before casting) and joining(2 &3) , stress relieved before final polishing.

Propeller testing after major repairs 

1.Pitch Measurement check

Local pitches are checked at 0.5R,0.6R , 0.7R ,0.8R , 0.9R and 0.95R , with measurements performed at two points on every blade. This test is to ensure no vibration during service , on account of variable pitch.

2.Balancing - Static 

Propeller is rotated  by inserting a special arbour into the bore and placed in a stand.

This test is to ensure that the propeller remains static in any rotating position.Weights are added and removed to achieve the same.

Assembly After Repairs:

This assembly follows satisfactory inspection and fitting of Tail shaft  in place after removal/ repair/ inspection as the case maybe ,  Stern tube bushing/ bearing jobs completed and fitted in place and Liner of simplex seal in place , Stern tube seal assembled , and rope guard fitted in place(Rope cutters to be installed later after assembling the propeller)

Assembly

Propeller bedded to tailshaft and jacked up to usual shop mark. The Pilgrim nut is then screwed on the shaft with the loading ring against the prop boss. With the lever operated, high pressure grease gun, grease is pumped into the inner tube inside the nut at around 600 bar, ( w.p. stamped on nut, not to be exceeded), the prop will be pushed sufficiently up the taper to give the required frictional grip. The pressure is then released and the nut is rotated until it is hard up against the aft face of the prop hub and locked, fair water cone then fitted. 
Push up length is checked by Dial gauge and calculated based on shaft temperature and Von-Meiss stress criterion as per original designed condition(as per graph). Von-Meiss stress for maximum interference fit should not exceed 70% of Yield stress of the Propeller material.

The Lock plate has to be assembled , cone  has to be filled with tallow before assembly , then slots to be sealed after assembly with bolts and slots to be covered with  a special cement.